A Foreign Office team is also focusing on returning survivors to the UK, he said, after a raid by Algerian troops ended the four-day siege on Saturday.

Five Britons and one UK resident are feared dead or unaccounted for. One Briton has already been confirmed dead.

Algerian authorities say a total of 23 hostages and 32 terrorists have died.

The Algerian Interior Ministry also said that 107 foreign workers and 685 local employees had been released following the end of the hostage-taking by Islamist militants at the Western-run In Amenas gas facility, in the Sahara desert.

Speaking at the Foreign Office on Saturday evening, Mr Hague said: "Our priority now is to work out exactly what has happened to every British national who might have been caught up in this terrible situation."

He went on: "On the basis of all the information we have... we believe that there are five British nationals and one UK resident who are either deceased or unaccounted for, in addition to the one fatality that we had already confirmed.

"Now obviously we're working hard to get definitive information about each individual.

"We're in touch with all of the families concerned and we're working closely with BP and with police forces across the country to give those families the support they need at this very difficult time."

Offers of help

Prime Minister David Cameron said that he had been told in a phone call from his Algerian counterpart when the hostage crisis was over.

"There is no justification for taking innocent life in this way," Mr Cameron said.

The In Amenas gas field is situated at Tigantourine, about 40km (25 miles) south-west of the town of In Amenas and 1,300km (800 miles) south-east of Algiers.

The plant is jointly run by BP, Norway's Statoil and Algeria's state-owned oil company.

Asked whether he felt regret that Algeria had refused British offers of help in tackling the militants, Mr Hague said: "They are a sovereign country. That is up to them."

While it was "right for this country to offer our help", he said he didn't "want to at this stage enter into criticism or judgement because there will be a lot to be learnt yet about this operation".

Conservative MP Patrick Mercer, a former homeland security adviser, suggested that no British special forces could have been in place "particularly quickly" to offer help.

"I think that we have the most capable special forces in the world, but what we don't know is how quickly the Algerians needed assistance, or how quickly they needed to move. My guess is that rapidity was incredibly important, and that's why they did what they did."

Former Liberal Democrat leader Sir Menzies Campbell said: "If it's a question that without military action being taken then there would be substantial loss of life then it's quite hard to argue with that."